1. Field of the Invention
The present invention relates to semiconductor manufacturing in general, and in particular, to a method of plasma treating bonding-pads on a wafer in order to prevent bond-pad staining problem which unfavorably affects the wire bonding.
2. Description of the Related Art
In conventional semiconductor manufacturing, the device structures that are formed on a wafer are sealed from environmental contaminants and moisture by chemical vapor depositing (CVD) on the wafer a passivation, or a protective layer, such as CVD phosphosilicate glass (PSG) or plasma-enhanced CVD silicon nitride. This passivation layer also serves as a scratch protection layer. Openings are then etched into the passivation layer so that a set of underlying special metallization patterns are exposed. These metal patterns are normally located in the periphery of the integrated circuits on a wafer as shown in FIG. 1a, and are called bonding pads. Bonding pads (40) are typically 100.times.100 micrometers (.mu.m) in size and are separated by a space about 50 to 100 .mu.m. FIG. 1b is a cross section through the bonding pads (40) of FIG. 1a formed on a wafer substrate (10) having metal and insulating layers (20), and an oxide layer (30) on which the bonding-pads are formed. Wires are bonded to the metal of the bonding pads and are then bonded to the chip package leads, as is well known in the art. A smaller pad (50) used for probing the wafer is shown in FIG. 1a for completeness, and passivation layer (60) which, initially, covers the whole surface of the wafer.
It has been found in the present state of manufacturing of CMOS devices, however, that when a color filter process is performed on the completed semiconductor substrate to form image sensors, the metal on the bond-pads is stained and pitted. The pitted metal, such as pitted AlSiCu alloy, then is found to cause a very poor quality wire bonding. A method of alleviating the bond-pad pitting problem by performing a judicious plasma treatment is disclosed later in the embodiments of the present invention.
Plasmas are generally defined to be a partially ionized gas composed of ions, electrons, and a variety of neutral species. In semiconductor manufacturing, they are usually used to perform etching of different materials, primarily silicon and oxides. As discussed more fully by Wolf, S., and Tauber, R. N. in "Silicon Processing for the VLSI Era," vol. 2, Lattice Press, Sunset Beach, Calif., 1990, pp. 542-543, usually, a glow discharge is utilized to produce chemically reactive plasma species (atoms, radicals, and ions) from a relatively inert molecular gas. These species diffuse to the surface of the material to be etched and are absorbed on the surface. Then, a chemical reaction occurs, with the formation of a volatile by-product where the by-products is desorbed from the surface and carried away by the bulk gas in the plasma.
The events that take place at the surface to produce etching of the solid material by the gas phase species involve mechanisms that relate to, among other things that are listed in Wolf, p. 545, the synergistic effects on the surface of multiple species bombardment by ions, electrons and photons, and are referred to as heterogeneous surface reactions. These heterogeneous reactions are governed by the nature of the surface that is being exposed to the plasma. Thus, etching oxides is different from etching metals. Similarly, plasma treatment of different materials yield different results, such as found with the favorable results attained with a plasma treatment of the bonding pads of this invention as disclosed later in the embodiments.
The mechanism of plasma etching, or plasma treatment in general, can in general be explained by the so-called fluorine-to-carbon ratio model, or F/C model, which is one of the two models that have been developed. Without going into the details that can be found in Wolf, the F/C ratio is the ratio of the fluorine-to-carbon species, which are the two "active species" involved in the etching of Si and SiO.sub.2 --as well as other materials etchable in fluorocarbon plasmas, including silicon nitride, Si.sub.3 N.sub.4, titanium, Ti, and tungsten, W (but not the aluminum alloys that form the bonding pads). The F/C ratio model does not attempt to account for the specific chemistry taking place in the glow discharge of a plasma, but instead treats the plasma as a ratio of F to C species which can interact with the Si or SiO.sub.2 surface. The generation of elimination of these "active species" by various mechanisms or gas additions then alters the initial F/C ratio of the inlet gas. Increasing the F/C ratio increases the Si etch rates, and decreasing the F/C ration lowers them.
Following Wolf, a pure CF.sub.4 feed gas, for example, has an F/C ratio of 4 as evident from its formula. If the plasma environment causes Si etching, however, this phenomenon consumes F atoms without consuming any carbon, and thus the F/C ratio is reduced. If more Si surface is added to the etching environment, the F/C ratio is further decreased, and the etch rate is also reduced. The addition of H.sub.2 to the CF.sub.4 feed gas causes the formation of HF, but does not consume any carbon, thereby the F/C ratio and the etch rate are again reduced. Finally, the utilization of gases in which the F/C ratio is less than 4, such as CHF.sub.3 or C.sub.3 F.sub.8, also has the effect of producing an F/C ratio smaller than that present in a plasma of pure CF.sub.4. Plasmas in which the F/C ratio is decreased to less than 4, are termed fluorine-deficient plasmas.
Conversely, the addition of O.sub.2 has the effect of increasing the F/C ratio, because the oxygen consumes more carbon (by forming CO or CO.sub.2), than F atoms (by the formation of COF.sub.2). Other feed gases that can be added to increase the F/C ratio include CO.sub.2, F.sub.2, and NO.sub.2.
In prior art, plasma treatment or plasma etching of various types of thin films have been developed. Thus, fluorocarbon-containing plasmas can be used to etch SiO.sub.2, and selectivity with respect to Si can be obtained by using fluorine-deficient plasmas. Nitrides deposited by plasma-enhanced CVD can be etched in CF.sub.4 -oxygen plasmas. On the other hand, both fluorine and chlorine based plasmas are used for etching polysilicon. As for refractory metal silicides and polycides, a multi-step etch process involving both chlorine and fluorine plasmas may be utilized.
However, as is well known in the art, fluorine, or F-containing plasmas are not suitable for etching aluminum since the etch product, AlF.sub.3, has a very low vapor pressure as shown in FIG. 18 of Wolf, p. 559. Chlorine based gases are instead used because the AlCl.sub.3 halide of Al has sufficiently high vapor pressure to enhance desorption from the etched surface. Alloys of aluminum, such as aluminum-copper, or aluminum-silicon-copper exhibit higher degrees of difficulty in etching with increasing copper concentration. Here again, F-containing plasmas are not effective. However, it is shown later in the instant invention that by a judicious use of F-containing plasma, the aluminum-silicon-copper metal of bonding pads can be protected from the detrimental effects of color processing performed in forming CMOS image sensors.
In related art, Mautz, et al, in U.S. Pat. No. 5,476,816 disclose a metal etch processing sequence which eliminates the need to use an organic masking layer solvent and etches a portion of an insulating layer after the plasma etching step. The etch of the insulating layer is performed with an etching solution that may include 1,2-ethanediol, hydrogen fluoride, and ammonium fluoride. On the other hand, Imai, et al., in U.S. Pat. No. 5,716,494 disclose a dry etching method, chemical vapor deposition method, and apparatus for processing semiconductor substrate where XeF.sub.2 gas is used as a process gas. A protective film is formed on the surface of a substrate to improve the profiles of an opening. An alcohol containing plasma is used by Tabara in U.S. Pat. No. 5,451,293 in making a wiring layer. An Al based wiring material layer is formed on an insulating film covering the surface of a semiconductor substrate. The wiring material is then selectively etched by a Cl base gas by using a resist layer as a mask, to form a wiring layer. The resist layer is ashed by using a plasma of a mixed gas of an H-and-O containing gas and a F containing gas, such as O.sub.2 /CHF.sub.3 /CH.sub.3 OH, without heating the substrate. Finally, Lee in U.S. Pat. No. 5,334,332 discloses cleaning compositions for removing etching residue from substrates containing hydroxylamine and at least one alkanolamine.
Also Harada in JP 03-136,240 and Kinoshita, et al., in JP59-132,622 show aluminum etch methods using F-containing gas plasma that form AlFx protective layers over the aluminum lines.
The plasma processes and the cleaning procedures of prior art fall short of addressing the problem of staining and pitting of bond-pads of semiconductor substrates subjected to color processing performed in forming image sensors. A plasma treatment which alleviates this problem is disclosed below in the embodiments of the present invention.